1. Field of the Invention
This invention pertains to a method for mounting multiple photopolymerizable printing elements onto a cylindrical element, and particularly a method for mounting flexographic printing elements each having a laser radiation-sensitive layer onto a cylindrical drum.
2. Description of Related Art
Photopolymer printing plates are well known for use in flexographic printing, particularly on surfaces which are soft and easily deformable, such as packaging material, i.e., cardboard, plastic films, etc. Flexographic printing plates can be prepared from photopolymerizable compositions, such as those described in U.S. Pat. No. 4,323,637. The photopolymerizable compositions are used to form photosensitive elements, and generally comprise an elastomeric binder, at least one monomer, and a photoinitiator. Photosensitive elements generally have a photopolymerizable layer interposed between a support and a coversheet or multilayer cover element. Upon imagewise exposure to actinic radiation, polymerization, and hence insolubilization of the photopolymerizable layer occurs in the exposed areas. Treatment with a suitable solvent removes the unexposed areas of the photopolymerizable layer leaving a printing relief that can be used for flexographic printing.
Imagewise exposure of a photosensitive element requires the use of a phototool which is a mask having clear and opaque areas covering the photopolymerizable layer. The phototool prevents exposure and polymerization in the opaque areas, and allows exposure to radiation in the clear areas so that these areas polymerize and remain on the support after the development step. Recently the need for a separate phototool has been eliminated by the availability of photosensitive elements that include a laser radiation sensitive layer covering the photopolymerizable layer.
U.S. Pat. Nos. 5,262,275 and 5,607,814 disclose a photosensitive printing element having a laser radiation sensitive layer and method for forming a flexographic printing plate from the photosensitive element. The photosensitive printing element includes in order a support, at least one photopolymerizable layer, a barrier layer, a laser radiation sensitive layer, and a coversheet. EP 741 330 also discloses a photosensitive printing element having a laser radiation sensitive layer which includes in order a support, at least one photopolymerizable layer, a laser radiation sensitive layer, and a coversheet. After removing the coversheet from the photosensitive element, the laser radiation sensitive layer is imagewise ablated from the photosensitive element to form an in-situ mask on the element. Thereafter, the photosensitive element is blanket exposed through the mask and treated to remove the unpolymerized areas and form the relief.
U.S. Pat. No. 5,760,880 discloses an apparatus for imagewise exposing a laser radiation sensitive layer, and specifically imagewise ablating the infrared radiation sensitive layer from the photosensitive element. The photosensitive element is mounted onto a rotatable cylindrical surface, i.e., a drum, for the element to be exposed to the laser radiation. The drum rotates at speeds from 200 to 2000 rpm during exposure.
In use, considerable time and effort are involved to manually mount a photosensitive element onto the drum of the laser radiation apparatus, place the drum in the apparatus and align the element for proper exposure. For some printing jobs, such as flexible packaging, tag and label, and bag and sacks, only strip portions of a photosensitive element are needed. For other printing jobs, a flexographic printing plate is often assembled from individual pieces. Such assembly might be required, for example, by the subject being printed, for reasons of economy, or to achieve a larger plate format. In both printing jobs, the width of the strips or portions of the photosensitive element is less than the full width of the drum. So, it is desirable to maximize the productivity of the laser radiation exposure apparatus by occupying the entire drum surface with one or more photosensitive elements.
Further, as each successive photosensitive element is mounted onto the same cylindrical drum surface for use in the laser radiation exposure apparatus, the elements should be mounted with the same considerations with which printing elements are mounted onto printing cylinders, such as balance, registration, and absence of trapped air bubbles. It is desirable to eliminate manual mounting of the photosensitive elements onto the drum and take advantage of mounting technology used in apparatus for mounting flexographic printing plates onto printing cylinders. It is also desirable to avoid mistakes in registration which can occur with conventional mounting methods.
Prior to mounting the photosensitive element onto the cylindrical drum surface, the coversheet is removed from the photosensitive element. Otherwise, if the coversheet is kept with the photosensitive element, the coversheet will shift as the element is secured to the drum and will damage the laser radiation sensitive layer. However, after removal of the coversheet, the laser radiation sensitive layer of the photosensitive element is very susceptible to contamination by dust, and to damage, particularly if touched or handled. So it is important to maintain the integrity of the laser radiation sensitive layer for each photosensitive element as each successive photosensitive element is mounted onto the cylindrical drum.